Making the Power-5

Thanks Trevor, interesting read

Stafford bike show today.... need to replenish my junk collection

Blowdown or Total Port Area, and a few other things?

“There is nothing to be gained by calculating the total exhaust port T/A, for it is insignificant compared to that of the blowdown T/A”!
I saw this statement from a well-respected 2t man a while back on one of the European sites I regularly visit and it made an impact, and so got me thinking about that whole proposition, and yes, perhaps we should be using the cylinder blowdown as the basis for our exhaust calculations. Blowdown has a direct and significant relationship with an engine`s power potential. We might also conclude that using total exhaust port area is pretty irrelevant and perhaps misleading and could even be an impediment to finding a more efficient exhaust port/duct/pipe flow regime?
 Blowdown is measured as the time and port open area prior to the pressure equalising between the cylinder and crankcase/transfer ducts such that will enable fresh mixture flow into the cylinder.
Fundamental to this is fully understanding that anywhere within an engine which relies on gas flowing, that the flow cannot be initiated in the desired direction until pressure between the two chambers is equal! That mixture then will then begin to accelerate out of the transfer ducts, as the rate of acceleration ceases the mixture velocity will have reached its maximum value.
There is a certain ambiguity associated with blowdown, it starts at the crank angle where the exhaust port effectively begins to flow, but where does it finish? It should be when the transfer ports first open, but as revs rise there is a reducing time frame for this to happen, consequently blowdown pressure may carry on past transfer port opening and those ports then become convenient, additional exhaust ports. Should this occur then a form of chain reaction ensues, bringing adverse effects with it.
Hot exhaust gas entering the transfer ducts at pressure will delay mixture transfer, it will also preheat some that cooler mixture. When cylinder pressure has dropped sufficiently enough to allow transfer to begin, that old useless exhaust gas has first to be expelled, thus further reducing transfer time/area until real transfer starts. If this mixture has heated up whilst stationary in its duct then it will have expanded and have reduced in density and oxygen content, none of which is helpful to power production.
Flow reversal can occur when the cylinder is being pressurised by both the rising piston and early plugging pulse return, along with the case being de-pressurised by the rising piston. Strangely the same effect would happen even with a stationary piston and the transfer still open, it’s the Helmholts factor again moving gas from one vessel to another with the change of pressure sign, every-thing stops, and then reverses!
There are also valid reasons why revving the nuts off your engine that may have insufficient blowdown t/a will lead to rapid power drop off after its natural maximum rpm for peak torque, for all of the reasons stated!
Gas flowing within a system will always take the line of least resistance, and that is invariably not the intended direction and is true both inside the engine, here in the case of scavenge flow, and outside where cooling air flow is involved, the negative consequences of which can be quite significant.  
The real dilemma we have is that there will be no precise indication of the crank angle where cylinder and transfer pressures equalise that will initiate scavenge flow?
In an ideal world with a race engine running at peak torque that has sufficient cylinder blowdown there should be no exhaust flow at transfer opening. If that should be case then there can be no argument that says the whole exhaust port area needs to be regarded as being relevant, this then supports the proposition the exhaust event is all but over by something less than 2/3 of port open duration.  
Blowdown T/A is the product of port angle area divided by rpm, if the cylinder port remains unchanged and there is no power valve installed, the time-area varies only with rpm, irrespective of case pressure. If case pressure was increased it would mean that cylinder pressure has to drop less to create equal pressures. Put another way, a higher crankcase pressure requires less blowdown time/area!  
 Over the last few years of two stroke race engine development the overall exhaust T/A has reduced because the overall profile of the port window has changed. There now is less open area in the cylinder wall from transfer port roof down to bdc. Time/area is purely a mathematical calculation which cannot make any direct or realistic account of flow-coefficient potential.
The current thinking now is to lift the port window and duct floor from bdc, this has the multiple benefit of reducing problematical short circuiting of fresh mixture from the adjacent transfer port window straight into the exhaust duct. Reduction of duct volume in turn provides for stronger exhaust pulses, lower turbulence losses, hence less mixing of fresh and spent gasses, all of which will improve blowdown out-flow coefficient.
Integral to all of this are duct outlet diameters, or more relevantly duct volume/outlet areas, the following published quote from Professor Blair is particularly revealing.
“In a racing pipe, where maximising the plugging pressure gives the highest trapping efficiency there seem little point in having the first downpipe diameter much more than 1.05 times larger than the exhaust port flow area, for in unsteady gas flow, the larger the pipe the lower the pressure wave amplitude for the same mass rate of gas flow”.
This being the case then exhaust duct outlet areas/diameters in current use might now be thought to be excessive. It is worth looking again at the published data from George`s 56 x 50.5 engine. With its exhaust port timing duration of almost 200*.The total port area, assuming a typical 10mm corner radii, is some 841.5 sq mm which equates to a diameter of 32.7 mm. So there is a difference of 1.16 from the 38mm diameter of the port/pipe shown. Working through further, the area ratio becomes 1.34. This situation can be adjusted if the stated port width of .625 of chord is moved to .70 chord, a very typical percentage for modern a race engine, the diameter difference then becomes a little more favourable, but not ideal, 1.09. Adjusting the width also and usefully increases blowdown angle/area. However, even 1.05 is excessive by 21st century standards, perhaps another example of ` the moving hand having writ, then moved on`! When now calculating the duct outlet diameter then 90% of the effective area can be used.  
I always find it a little strange that the tail pipe diameter was once felt to be a function of the exhaust port total area, and not on the basis of the quantity of exhaust gas generated by the engine that it has to handle. The quantity of exhaust gas created is proportional to the amount of power the engine in question is actually producing, and that is only true between the rpm of max torque and max power.    
All of these numbers are just that, merely numerical opinions but the back drop to this is the very long and ultimately limiting 198/200* port duration so symptomatic of very short stroked, 2t engines with the inevitable loss of performance at lower rpm.
The following story I think serves to illustrate some of the points being made and is a perfect example of the tuner, both knowing, and doing his stuff then getting the desired results.
The experiments were performed on an aftermarket “Luyten” barrel for the Honda 125RS engine. As supplied the oval exhaust duct outlet had dimensions of 41 x 36mm, the engine was dyno`d and results recorded. The duct was then over-bored and an insert installed with revised dimensions of 41 x 32mm, dyno result + 1.5hp. Finally a new 25 mm long spigot was internally CNC cut to transition from the 41 x 32 out to the 41mm diameter of the header pipe with no step which the original Honda spigot had with the barrel. Result, another 1hp, so for simply reducing the duct volume and outlet with an insert and new profiled spigot, a full 2.5 hp was gained throughout the power range and so conclusively proved the theory!
Placing historical and preconceived notions about exhaust ports and outlets to one side, the concept and premise outlined in this article is potentially one of…… less is more…. but there is always more going on than ever meets the eye?

Just few thoughts to put your way to get the Bantam performing just that bit better, but all things considered, what`s not to like?

Trevor.

This transforms my thinking about the Walsh Bantam and its fascinating run of success. One has to ask sevearl questions:  what was the competition like in those post war days? and what alcohol/leaded fuel was the Walsh Bantam using?

The articles I have read on the Welsh Bantam suggest -- with its short megaphone pipe one time and longer pipe another but not resonant by any means  --  and having no rev counter but it revved on and on and lifting the float chamber a quarter of an inch by an air lever on the handlbars  caused it to rev 3 thousand and a bit more--  more -- was more that it was on a high compression ratio using methanol or some such doped fuel, had one 60thou thick ring which was changed between practice and racing and had very low friction losses thro' out....

   AND --- what of the 250s competition in those post war years of 4-stroke 250s ? Not many would reliably stand a lifting of the compression ratio above 10:1.... Saw plenty of disasters  in my grass track days where I had a fairly quick 1937, 3valve, 250 AJs  using methanol with 9.5CR and good relaibility.

I imagine the situation would be the same in Australia in those days with road racing  --  not many well-tuned 4 strokes ?

Not to downgrade Walsh or any of the other Aussy tuners but I think having a regulation that allowed alcohol and tetra-ethyl leaded fuels was probably more a theortical (air-stanadard) thermal efficiency clue to its success than anything to do with port tuning in those days.

Don´t hesistate to correct me, Trevor: certain aspects you have mentioned as supporting tuning for power were definite No-No´s by two-stroke tuners I knew in the late sixties- early seventies:  Said was,  no no, you cannot increase blown-down beyond xxyy  degrees -- which proves the point that many hadn't the dyno facility in those days and relied on Nerks like me to return from Barnds parctice to make their changes.  

  Besides,  I am only looking at the theoretical comparison of 9:1 CR with a good (say 98 octane) petrol  and a 16:1 CR using  leaded alcohol (can be around 160 octane) combined with very-very low dragging resistance in all components and lubrication of engine and cycle parts....   9:1 to 16:1 compression ratio theoretically increases power by 9% which is quite an eye-popping gain for any tuner.

Instances like changing a phospher bronze bush for a ball-bearing -- cán`t make that much difference, can it? -- The other way around when considering the square law factor relative speed when, say, increasing max  revvs  from 8.400 to 12,500rpm looks quite a bit different.....

Time to go back to sleep.....

Schnasrsch!

Cheers!

JayBee.

Hello John,
You are quite correct John, so I won`t be attempting to correct you on the historical attitude towards blow-down and the need back then to observe certain limits, everything in those engines was at the outer limits of common sense! We however, have the benefit of hind sight and out to be able take advantage of current technologies?
You and I are lucky in that we lived through that era and grew alongside the emerging technologies, however a good deal of that stuff has now proven to be misleading and confused.
Back in the day most two stroke thinking was for as small a crankcase volume as possible, way up over 1.7, very high transfer ports, in some cases of over 140*transfer duration and exhaust timing over 200*, Suzuki even had a small port above the exhaust port that fed and pressurised the case prior to transfer opening. Needless to say the little works air-cooled 50cc engine was so prone to indiscriminate piston seizure that the idea was quickly scrapped. The upshot of all of this extreme tuning was the need for a dozen or more gears to keep engine revs within the miniscule power band. Extending the blowdown on top of a long transfer timing just makes everything so much worse. When you think that the best of the 125 GP engines of today make more power than a 250 of the 60/70s then a lot was wrong back then. The engineers had their time and the scientist/engineers took over, and they made the difference.

Trevor

Thanks Trevor...

I was married, had a wife, two kids and a house to attend to so I was quite happy to let Derek Neil look after the engine side of things. In any case it was only because of Derek and his brother Colin -- a Bantam Senior -- who worked for Dr Joe Erhlic that I´d ever have got thinking of road racing and on a Bantam at that. Derek had an immaculatrer, homemade 250 Villiers he and another rider shared which was used by Peter Williams during National & International meetings to fill in the time spaces between his *Works" sponsored rides and Peter often finished in the first three with the Neil 250...
I think Derek was more thinking of my body weight when tuning than going with other development trends of the day and Icarus-1 had a wider torque band than many others of the day. Calculated from my telling of revs before and after max revs achieved at certain points of circuits -- particularly Brands practice -- to arrive at the conclusion of the power of Icarus-1 being around 12.5 to 13. bhp at 8400rpm and the power band width to be around 3000rpm, When Derek gave up on me I made up another racing Bantam and did all but the porting which Wobblyman (Colin Aldridge9) did for me that allowed Icarus-2 to rev to 11000rpm with alarmingly poor reliability mostly down to using standard BSA pistons. This was when I became convinced that reduction of friction losses were so very important in engine tuning. The crankcase seals would be so "loose" as to be almost ineffective, piston gudgeon pin holes would be oval and the noise alarming as the piston crown was thunking the head but the acceleration was fantastic and the thrill of passing a "Senior Ace" -- on the straight:. exhilertaing -- which feeling has never left me, although poor Icarus-2 gave up the ghost a short time after...
Andy Boyle´s 250 Alpaha changed all that and Icarus-i and Icarus-2 -- if and when avaialable, i must confess -- became fillers-in between 250 rides.

In a way dissatisfied --- I feel I never finished what I had started -- or perhaps it was that I didn't really ever know what it really was, that I had got into. ...
Certainly when Wobblyman conned me into attending committee meetings at the Hoop & Grapse in Holborn I
never visualised myself as Chairman of the Bantam Racing Club.

Ever with me -- 44 years ago now, in a couple of days -- was my most successful motorcycleing racing day: 19th May 1973, at llandow -- when it rained during the first race and carried on during the whole day and the next three races I was in... Only Intermediate races, of course, so nothing to crow about....
Unfortunately, before that, Wobbleyman came off in practice (someone started practice off before sheep droppings were cleared) and he later (in the day) rode Icarus-2 while suffering from concussion (Wobbley, the sufferer I mean) and nearly won a senior race but for the Icarus-2 non-relaibility factor and Niffy Goebells*** boke his leg in a Novices race which necessitated my driving to the other (Western) end of Wales (Bridgenorth hospital, I think?) before driving his Ford and tailer back to London.

For goodness sake? -- what brought that on?
I suppose it was because of how badly prepared Icarus-2 was compared with Icarus-1 and the 250 Alpha...

... siómething like that!

*** Niffy died after crashining during the Manx -- I forget which year..... The East Enders contributed to a memorial on the circuit near where he crashed -- was some sort of convenience or shelter, or something!

Cheers!




.

Hi John B.

There is a Wikipedia page which claims to list all fatalities of competitors on the TT mountain circuit. According to this ( https://en.wikipedia.org/wiki/List_of_Snaefell_Mountain_Course_fatalities ) your mate "Niffy" Gobell crashed at Quarry Bends on 29th August 1994, while competing in the MGP Senior Classic race. He was entered on a 492cc Weslake.

The list is very long, and makes sobering reading. So many of the names are familiar to me.

Another intresting read Trevor. plenty to consider

Thanks John C...
I was in Africa when it happened and it was quite a shock to come home to...

It is sometimes difficult -- NO! I'll say it WAS sometimes awkward -- with the type of people I mixed with professionally (some time ago now) to mention an involvement with motorcycle racing because these people were always car types, or worse still, racing-car types who only knew of the Hell`s Angels or Rockers and considered motor cycle owners as a definite class below them: lovers of titles like "Prof, Dr...Dr..." (Profesor, Doctor, Doctor) all in one row after the name is not unusual which we see often here in Deutchland. Yet there were and still are some fantastic German road racers who only get mentioned when the national superiority needs a bit more support.

The contract I had to work in Africa (for expenses -- no salary or wages): Malawi for example, stated I was to be provided with personal transport and the Chief Clerk ( a joking Brit expatriate) in Blanytyre said nothing doing, we have no spare vehicles, we are even short of Sewage Tankers and I said I´d seen motorcycles with the Blantyre shield on the tank ... So it was I had a Honda 100cc with five gears and experienced the surprise of a performance better than previous British, 350 and 500cc road hacks I'd had yonks before. Then, of course, the rains came. There´d been a long drought up to then!

The Niffy memorial is, I believe, a bus sheler but some East End joker called it "A Convenience" and somehow that did the rounds for a while....

Cheers!

JayBee.

Just an inch away from disaster, some prevail, some become another victim and so are added to the statistical role call!
This is a breathtaking image of Michael Dunlop on the Island, I`m in awe of these incredible riders but fear of serious injury and possible death would prevent me from even trying.
Very sad about Niffy, John, a bus shelter seems a meagre memorial for a brave competitor, he gave to the Island but it took him anyway!  

Cheers, to all Trevor

Yes Trevor you are right -- but the Eastern Centre lads collected enough among their small group to make it possible. He is also remembered for his help to my family when we were struggling with Canada and its difficulties and the way he was always prepared to help others.

I could never iamagine myself road-racing -- seemed far too dangerous -- but somehow it happened.....Grasstrack and Speedway practice seemed about right ....

Take care,
Cheers,

John-Boy....

hey mates, and thaks for this topik it was verry usefulL! used to be a racer myself but i had an accident which made me be an active me on such as https://pharmacyreviews.md but it seems that i'm now ready to go so here i am here back again.

You are welcome Youty1983 -- but why aren´t you spouting on Facebook with the rest of them?

I managed to get a look in on, on an occasion, about a year back. That site seems to have more of the active racers spouting than on here. Then my password was rejected and the new one Facebook gave me was again rejected.

Perhaps someone on that site has the Power to have me rejected -- hence Making the Power fits ....

Point I am trying to make is that we contributors -- on here -- are thin on the ground and I wonder how long we shall last.

Your "Mates" expression is suggestive of Australia. Youty -- and how do you explain "Youty?"

Just being Nosey ...


All tzhe best.

JayBee for John-Boy ex Chairman `72 thro´to '74....